This invention relates generally to implantable medical devices. More particularly, the present invention relates to methods and apparatus for utilizing an electrode assembly having an array of electrodes as part of a baroreflex activation device.
Cardiovascular disease is a major contributor to patient illness and mortality. It also is a primary driver of health care expenditure, costing billions of dollars each year in the United States. Hypertension, or high blood pressure, is a major cardiovascular disorder that is estimated to affect 65 million people in the United Sates alone. Of those with hypertension, it is reported that fewer than 30% have their blood pressure under control. Hypertension is a leading cause of heart failure and stroke. It is the primary cause of death for tens of thousands of patients per year and is listed as a primary or contributing cause of death for hundreds of thousands of patients per year in the U.S. Accordingly, hypertension is a serious health problem demanding significant research and development for the treatment thereof.
Hypertension occurs when the body's smaller blood vessels (arterioles) constrict, causing an increase in blood pressure. Because the blood vessels constrict, the heart must work harder to maintain blood flow at the higher pressures. Although the body may tolerate short periods of increased blood pressure, sustained hypertension may eventually result in damage to multiple body organs, including the kidneys, brain, eyes and other tissues, causing a variety of maladies associated therewith. The elevated blood pressure may also damage the lining of the blood vessels, accelerating the process of atherosclerosis and increasing the likelihood that a blood clot may develop. This could lead to a heart attack and/or stroke. Sustained high blood pressure may eventually result in an enlarged and damaged heart (hypertrophy), which may lead to heart failure.
Heart failure is the final common expression of a variety of cardiovascular disorders, including ischemic heart disease. It is characterized by an inability of the heart to pump enough blood to meet the body's needs and results in fatigue, reduced exercise capacity and poor survival. Heart failure results in the activation of a number of body systems to compensate for the heart's inability to pump sufficient blood. Many of these responses are mediated by an increase in the level of activation of the sympathetic nervous system, as well as by activation of multiple other neurohormonal responses. Generally speaking, this sympathetic nervous system activation signals the heart to increase heart rate and force of contraction to increase the cardiac output; it signals the kidneys to expand the blood volume by retaining sodium and water; and it signals the arterioles to constrict to elevate the blood pressure. The cardiac, renal and vascular responses increase the workload of the heart, further accelerating myocardial damage and exacerbating the heart failure state. Accordingly, it is desirable to reduce the level of sympathetic nervous system activation in order to stop or at least minimize this vicious cycle and thereby treat or manage the heart failure.
A number of drug treatments have been proposed for the management of hypertension, heart failure and other cardiovascular disorders. These include vasodilators to reduce the blood pressure and ease the workload of the heart, diuretics to reduce fluid overload, inhibitors and blocking agents of the body's neurohormonal responses, and other medicaments.
Various surgical procedures have also been proposed for these maladies. For example, heart transplantation has been proposed for patients who suffer from severe, refractory heart failure. Alternatively, an implantable medical device such as a ventricular assist device (VAD) may be implanted in the chest to increase the pumping action of the heart. Alternatively, an intra-aortic balloon pump (IABP) may be used for maintaining heart function for short periods of time, but typically no longer than one month. Cardiac resynchronization therapy (CRT) may be used to improve the coordination of the heart's contractions. Other surgical procedures are available as well.
It is known that the wall of the carotid sinus, a structure at the bifurcation of the common carotid arteries, contains stretch receptors (baroreceptors) that are sensitive to the blood pressure. These receptors send signals via the carotid sinus nerve to the brain, which in turn regulates the cardiovascular system to maintain normal blood pressure (the baroreflex), in part through activation of the sympathetic nervous system. Electrical stimulation of the carotid sinus nerve (baropacing) has previously been proposed to reduce blood pressure and the workload of the heart in the treatment of high blood pressure and angina. For example, U.S. Pat. No. 6,073,048 to Kieval et al. discloses a baroreflex modulation system and method for stimulating the baroreflex are based on various cardiovascular and pulmonary parameters.
Another method of treating hypertension comprises electrical stimulation of the baroreceptors, a practice known as baroreflex activation therapy. U.S. Pat. No. 6,522,926 to Kieval, et al. discloses a baroreflex activation system and method for activating baroreceptors to regulate blood pressure. Implantable electrode assemblies for electrotherapy or electro-stimulation are well known in the art. For example, various configurations of implantable electrodes for an implantable baroreflex activation device are described in U.S. Published Application No. US 2004/0010303A1. One type of electrode assembly described therein is a surface-type stimulation electrode that includes a set of generally parallel elongate electrodes secured to, or formed on, a common substrate or base typically made of silicone or similar flexible, biocompatible material that is designed to be wrapped around and then typically sutured to the arterial wall. Prior to implantation in a patient, the electrodes are generally electrically isolated from one another. Once the electrode assembly is implanted, one or more of the electrodes are utilized as a cathode(s), while one or more of the remaining electrodes are utilized as an anode(s). The implanted cathode(s) and anode(s) are electrically coupled via the target region of tissue to be treated or stimulated.
The process of implanting the electrode assembly involves positioning the assembly such that the electrodes are properly situated against the arterial wall of the carotid sinus, and securing the electrode assembly to the artery so that the positioning is maintained. One example of mapping methods and techniques for implanting electrodes is disclosed in U.S. Pat. No. 6,850,801 to Kieval et al. The positioning is a critical step, as the electrodes must direct as much energy as possible toward the baroreceptors for maximum effectiveness and efficiency. The energy source for the implanted baroreflex stimulation device is typically an on-board battery with finite capacity, and it is desirable to provide a lower energy source to ensure patient safety. A high-efficiency implantation will provide a longer battery life and correspondingly longer effective service life between surgeries because less energy will be required to achieve the needed degree of therapy. As such, during implantation of the electrode assembly, the position of the assembly is typically adjusted several times during the implantation procedure in order to optimize the baroreflex response.
This process of adjusting and re-adjusting the position of the electrode assembly during implantation, known as mapping, adds to the overall procedure time. Present-day procedures involve positioning and holding the electrode assembly in place with forceps, hemostat or similar tool while applying the stimulus and observing the response in the patient. Movement by as little as 1 mm can make a medically relevant difference in the effectiveness of the baroreceptor activation.
Another challenge related to the positioning process is the task of keeping track of previous desirable positions. Because positioning the electrode assembly is an optimization procedure, surgeons will tend to search for better positions until they have exhausted all reasonable alternative positions. Returning the electrode assembly to a previously-observed optimal position can be quite difficult and frustrating, especially under surgical conditions.
After determining the optimal position, the surgeon must secure the electrode assembly in place. In an existing technique as described in U.S. Published Application No. US 2004/0010303A1, this is accomplished by wrapping finger-like elongated portions of the electrode assembly around the artery and suturing the assembly in place. The electrode assembly can be sutured to the arterial wall or to itself (after being wrapped around the artery). Loosening or removing the sutures, re-positioning the electrode assembly, and tightening or re-installing the sutures can increase the time and costs associated with the devices, and can also increase the risk of complications or surgeon errors related to protracted surgical procedures and fatigue.
In one embodiment disclosed in U.S. Published Application No. US 2004/0010303A1, an electrode array is described in which electrical paths through the tissue of the carotid sinus may be selectively defined by one or more pairs (bipolar) or groups (e.g., tripolar) of electrode pads in a multi-channel electrode array that is described in one embodiment as a four-by-four array of electrode pads. The electrode pads of the multi-channel electrode array are described as selectively activatable for purposes of mapping and targeting a specific area of the carotid sinus to determine the best combination of electrodes (e.g., individual pair, or group of pairs) to activate for maximum baroreflex responsiveness. The process for determining the best combination of electrodes as described involves conventional stimulation testing with the pairs or groups of pairs being stimulated with varying test patterns and the effectiveness of the baroreflex responsiveness being determined for each combination. While effective, this process can become lengthy and involved as the number of combinations of electrode pairs or groups of pairs being tested increases or if the electrode array is repositioned along the carotid sinus.
A further challenge facing current implantable baroreflex treatment devices is power usage. Often, baroreflex therapy is used in conjunction with drug therapy to treat hypertension, with the goal of reducing a patient's dependence on medication. When a patient first begins baroreflex therapy while still on medication, the amount of power required to induce a favorable baroreflex system response is relatively low. As the patient's reliance on medication becomes reduced, there can be a tendency to increase the baroreflex therapy by increasing the power of the stimulation. However, this negatively impacts battery life.
Accordingly, there exists a need for a baroreflex treatment that could deliver electrical stimulation and may be more easily implanted, adjusted or modified.